1. Field of the Invention
The present invention relates to, for example, a radio wave receiving apparatus for receiving standard radio waves, a radio wave receiving circuit and a radio wave timepiece.
2. Description of the Prior Art
Recently, in various countries, for example, Germany, England, Switzerland, Japan and the like, time data, that is, standard radio waves containing a time code are being transmitted. In Japan, long wave standard radio waves of 40 kHz and 60 kHz with time codes which are amplitude modulated, using the standard time format, are sent from two transmitting stations (Fukushima and Saga). This time codes are sent with the frame the cycle of which is 60 seconds, for every occasion when the place of minute of the exact time is updated, that is, for every 1 minute.
In recent years, so-called radio wave timepieces have been commercially available, in which standard radio waves containing the above-described time-codes are received to adjust the current time. Radio wave timepieces receive standard radio waves through a built-in antenna and perform amplification, detection and the like of the received signal, thereby to decode the time code and to adjust the current time. Radio wave timepieces capable of receiving a plurality of standard radio waves different in frequency, which are so-called multiband radio wave timepieces are known.
The radio-wave receiving apparatuses used in multiband radio wave timepieces are mainly based on a super-heterodyne system in which a received signal is synthesized with a local oscillating signal having a predetermined frequency to convert into an intermediate-frequency signal (IF signal), and the intermediate-frequency signal is referenced to perform detection. In this case, it is necessary to provide a local oscillating signal having a frequency, depending on the frequency of the standard radio wave to be received.
Therefore, the constitutions therefor are known, namely, (1) a constitution in which a plurality of local oscillating circuits are provided for outputting a local oscillating signal corresponding to the frequency of the standard radio wave to be received, (2) a constitution in which one unit of a local oscillating circuit is provided and the oscillatory frequency of the local oscillating circuit is changed over by turning a received frequency selecting switch ON/OFF, and (3) a constitution in which a local oscillating circuit is used also as an oscillating circuit for measuring time and the frequency is divided by the frequency-dividing ratio corresponding to the frequency of the standard radio wave which receives a reference frequency signal of 32.768 kHz output from the oscillating circuit for timepieces, to give a local oscillating signal.
However, the above-described conventional multiband radio wave receiving apparatus has the following problems. Namely, in the case of (1) where a plurality of oscillating circuits are provided corresponding to the frequencies of standard radio waves to be received, a radio wave receiving apparatus is inevitably provided with a larger sized circuit, thereby resulting in an increased cost and a restricted frequency of standard radio waves that can be received, which is a problem. Further, in the case of (2) where a local oscillating circuit is provided, the local oscillating circuit is constituted with, for example, a PLL (Phase Locked Loop) circuit having a VCO (Voltage Controlled Oscillator) and the PLL circuit requires a certain time from power-on to a stable operation and an oscillatory frequency output from the VCO may be rendered unstable, depending on setting of the reference frequency, which is also a problem.
In addition, in the case of (3) where the reference frequency signal for measuring time is frequency-divided to give a local oscillating signal, the frequency-divided signal is not in perfect coincident with the frequency necessary for converting the frequency of a received signal into an intermediate frequency, thereby to make it impossible to perform an accurate detection due to a difference in frequency, which is also a problem.